Method of heat treating steel and the metallurgical product produced thereby



9 w is .mi .SAQ .5 um s m NN .N Q w m 15E: :z n .Dr/ 4, m Q S Qn u n m M lnlltwwlllv ihm, lmw mmm m 2.. H U N T D LNI IVH A M l IHQhN A T Il I l C m u -..H was N. m 11mm m u n n l M U w l* H H, T m m. l H v R U IMQW /l y O m` 5 l Im 0 w m t. l u M E M. Il N Ilhv D T n l S l s. m n u s I d E M 11190 m v m a H s I I m IIR i HS@ A m H m u 11.@ j l m d n w \I& m m |=E 52:.- 5:2. J u Immfmdf" UNM hardness distribution Patented June Il, 1940 VUNITED STATES PATENT oFFlcE 2,204,209 METHOD 0F HEAT TREATING STEEL AND THE METALLURGICAL DUCED THEREBY PRODUCT PRO- Edmund s. Davenport, Chatham, N. J., and Edwin H. Engel, Vandergrift, Pa., asslgnors to United States Steel Jersey Application October 5, 1938, Serial No.

12 Claims.

and that by an appropriate heat treatment subsequent to quench hardening we may obtain a reversal of the usual structure in a shallow hardened steel, thereby obtaining a steel product having a. hardened core and softened exterior which placed into service.`

Before further disclosing the present invention, reference should be made to the accompanying drawing wherein:

Fig. 1 illustrates in curves M, FP and P the rate of softening of martensite, ne pearlite and Acoarse pearlite, respectively, in terms of Brinell hardness number, when heat treated at the same tempering temperature, namely, 1200 F.

Fig. 2 illustrates in curves l, 2, 3 and 4 the across the diameter of a round bar in terms of Rockwell C hardness number (1) in its quench hardened condition; (2) after heat treatment for twenty seconds at F.; (3) after heat treatment for fty seconds at 1200 F.; and (4) after heat treatment for one hundred seconds at 1200 F.

Referring to Fig. 1 it may be noted that martensite (curve M) upon heat treatment at 1200 F. softens more rapidly than does fine pearlite (curve FP) or coarse eventualy becomes the softest of the three structures. On the other hand, the coarse pearlite 'Ihus it will be apparent from the curves Fig. 1 hat it is possible by heat treatment to alter the martensite formed by usual quench hardening methods into a. relatively soft product and where-there is formed by such quench hardening methods a core of either fine or coarse pearlite, the core by appropriate heat treatment may be rendered relatively harder than the originally hard martensitic surface. In ordinary quench hardening methods the martensite is formed in- Oorporation, a corporation of New wardly from the exterior surface a distance ordinarily controlled by the mass and composition of the steel and the rate of cooling applied thereto.

As an illustration of the present invention, reference should be made to the curves in Fig. 2. I'he steel composition of the 1%@ inch diameter 'in curve I (Fig. 2). Curve I shows the hardness distribution across the diameter of the steel bar. From the curve it may be noted that the center (C) of the round bar is considerably softer than the outer areas (A and B).

Upon heat treatment at 1200 F. for twenty seconds (curve 2) the hardness across the diameter chan-ged to an approximately equal value obtained at (C) curve I.l

Upon further heat treatment at the same temperature for a total of fifty seconds (curve 3) the exterior portion softened more rapidly than Heretofore it example, in the threading has been difficult, for of shaft or bolt ends that of curves I and 2.

The practice of the present invention is applicable with appropriate modilcations as to time and temperature oi heat treating which is readily discernable to those skilled in the art,

to a wide variety o1' steels and steel compositions known in the art as hardening steels wherein hardening is obtained by the formation of martensite through appropriate cooling from a temperature above the so-called critical range.

. rence of the transformation of austenite into ferritic structures on cooling the steel through the critical range. The rate of cooling of such steels must be adjusted with respect to the per-- centage of these alloying constituents present to yield the hardened martensitic exterior and pearlitic interior` desired for heat treatment in accordance with the present invention and the precise temperature of heat treatment of the hardened product must be selected withany given steel composition to yield within an economically practical time interval the desired conversion indicated in curves 3 and 4 (Fig. 12) One skilled in the art can readily ascertain these variable factors with any given steel composition.

The presence of these alloying constituents frequently is desirable to impart added strength, shock resistance and the like properties to 'the hardened core or to impart corrosion resistance to the surface of the metal.

Having broadly and specifically identified the nature and scope of the present invention and having given one specific embodiment of the same, it is apparent that many modifications and adaptations maybe made of the present invention without departing essentially from the nature and scope thereof as may be included within the scope of the accompanying claims.

What We claim is:

l. The method of treating a quench hardened steel having a hardened exterior portion consisting mainly of v'martensite and a softer interior core consisting mainly of pearlite to render the exterior portion softer than the interior core, which comprises heating the steel to a temperature at which the rate of softening of the exterior portion is greater than the rate of soften` ing of the interior core and maintaining the temperature until the exterior portion has acquired a hardness lower than the said core.

2. The method of treating a quench hardened steel having a marginal martensitic area and a central pearlitic core, which comprises heating the steel to a temperature at which the rate of softening of the martensite is faster than the rate of softening of the pearlite and maintaining this .temperature until the martensitic area acquires a hardness less than that of the pea-rlitic core.

3. The method of treating a steel having 'martensitic and pearlitic -zones or areas to substantially reverse the hardness 4relationship of said zones or areas, which comprises heating said steel to a temperature at which the rate of softening of martensite is faster than the rate of softening of pearlite and maintaining said transforming temperature for a time interval adapted to prduce a tempered martensitic structure which is softer than the simultaneously tempered pearlitic structure.

4. In the tempering of quench hardened steel, the step of heating said steel to a temperature at which the rate of conversion of martensite into tempered martensite is greater than the rate of conversion of pearlite into tempered pearlite and holding the steel at said temperature for a time interval effective to produce a tempered martensite softer than the tempered pearlite.

5. The method of treating a quench hardened carbon-iron alloy having an exterior area cornposed of martensite and an inner core area composed of pearlite, which comprises heating the same to a temperature at which the rate of softeningof the martensite is greater than the rate of softening of the pearlite and maintaining the steel at this temperature until the hardness of the martensitic area is less than that of the pearlitic core.

6. The method of treating a quench hardened steel containing carbon about .74% and having a marginal zone o1` area composed of martensite and an inner core composed of pearlite, which comprises heating said steel to a temperature approximating 1200 F. for a time interval in excess of about twenty seconds until the hardness of the said marginal zone is less than the hard"- ness of said core.

7. An article of manufacture consisting of a quench hardened and tempered steel article having a pearlitic core substantially harder than CII the surrounding tempered martensitic structure. 1 i

8. An article of manufacture consisting of a steel article quench hardened and tempered, said article having an outer portion consisting of tempered martensite of substantially lower hardness than the inner core.

9. An article of manufacture, a steel article provided withfan inner core area consisting of pearlite and an outer area. surrounding said core consisting of tempered martensite having a hardness materially less than that of the said core.

l0. An article of manufacture comprised of a transforming steel having a carbon content adapted to permit hardening of the same on being cooled from a temperature above the critical range to a temperature below said range, said article having an inner core area consisting substantially of pearlite and an outer area surrounding the said core area consisting of tempered martensite having a hardness materially less than that of the said core.

11. An article of manufacture comprised of steel, said article having a centrally located core consisting substantially of pearlite and an outer enclosing area consisting of tempered martensite having a hardness materially less than that of the core.

l2. An article of manufacture comprised of a steel having a carbon content and an associated alloy content adapting the steel to undergo transformation and hardening on being cooled through the critical rangeand a mass limiting the hardening to the outer portion of the article,

said article being characterized by a central core v v EDMUND s. DAVNPORT.

EDWIN H. ENGEL. 

